With a view to providing a work machine with a hydraulic fluid energy regeneration device and a hydraulic fluid energy recovery and regeneration device with space-saving dimensions and capable of enlarging the scope of recovered energy uses, there have been disclosed techniques involving a hydraulic pump motor driven by return hydraulic fluid from hydraulic actuators, an electric motor generating power when driven by the hydraulic pump motor, and a battery for storing the power generated by the electric motor (e.g., see Patent Document 1).